High-power microwave systems are used in many applications, including consumer, industrial, medical, and scientific sectors. In addition to the ubiquitous consumer microwave oven, high-power microwave systems are used to cook or thaw food products at an industrial scale, to dry many bulk organic and inorganic materials, to set resins in composite material, to create plasmas for semiconductor manufacturing, and many other industrial applications. Magnetron tubes have been used almost exclusively for over 50 years to generate high-power (from 1 to 100 kilowatts) microwave energy. Magnetrons are relatively efficient and cost-effective in conversion of line power to microwaves. On the other hand, industrial magnetrons have many disadvantages, such as: relatively short lifetimes (2000 to 6000 hours), high replacement cost; problematic reliability in continuous process applications; limited ability to control output power, limited or no possibility of modulation, being restricted to a fixed frequency, instability, including mode jumping, and use of dangerous very high voltages. Unlike some other higher power radio frequency (RF) applications, such as communications or broadcasting, high-power systems are usually continuous wave and no information is carried by the RF output. Most high-power microwave systems operate in the government allocated Industrial, Medical, and Scientific (ISM) bands. The bands most commonly used for microwave heating in the United States are 902-928 MHz (“L band”) and 2,400 to 2,500 GHZ (“S-band”). Allocations vary in other regions of the world.
Recent advances in semiconductor technology have made it practical and economically feasible to construct high-power solid-state microwave generators to replace magnetrons in many applications. The benefits of frequency agility possible with solid state devices have been known for some time, see e.g., MacKay, et. Al., U.S. Pat. No. 4,196,332, and Nobue, et. Al., U.S. Pat. No. 4,415,789. Laterally diffused metal oxide semiconductor (LDMOS) and gallium nitride (GaN) transistors are well suited for this application. Currently, no single transistor can deliver the power of a high-power magnetron, so the needed power output is achieved by using multiple power amplifier transistors operated in parallel with their outputs combined to produce a single power output. Functional blocks of two, four, or more transistors can be further combined in a hierarchy to achieve a system with very high output power levels. Solid-state microwave generators are in fact amplifiers, so their output frequency, phase, and amplitude can be varied in any combination. This gives solid-state microwave generators many useful capabilities which are difficult or impossible to obtain from magnetrons.
The main use of a microwave RF energy system is to apply high-power microwaves to some type of material, which may be a solid, liquid, or gas. Microwaves provide a unique form of volumetric heating. Often, but not exclusively, the purpose is to heat the material to cook it or dry it. Other uses include generation of plasma arcs, excitation of catalysts, and acceleration of chemical reactions, among others. The material being processed is preferred to be enclosed in some type of chamber which is mainly made of aluminum, copper, or stainless steel. The container can have many forms including a large box, a tank, a pressure vessel, or even just waveguide. One or more feeds or antennas are used to apply microwave energy to the process material. Within the industry, the generic term used for the processing chamber is called an “applicator”. Applicators are typically equipped with devices to move the material being processed, various sensors to monitor the process, and safety devices to prevent accidental exposure of microwave energy to people. The material being processed within the applicator is commonly called the “load”. The microwave energy source is normally connected to the load using a transmission line of waveguide or coaxial cable.
As with any RF system, maximum energy is transferred when the generator, transmission line, and load have matched impedances. The impedances of the generator and transmission line are defined and effectively constant. The load impedance is a complex conjugate which includes the electromagnetic properties of the applicator structure, the shape, mass, position, and dielectric properties of the load. These may all interact with each other. Often the characteristics of the load, and therefore the applicator impedance, will vary significantly during system operation due to changing properties of the load material.
When there is an impedance mismatch, electromagnetic waves are reflected from the applicator through the transmission line back to the generator, and standing waves appear in the transmission line. There are several detrimental effects of standing waves. First, there is a loss of efficiency. The reflected power will be absorbed by the transmission line and/or the generator. This energy is lost as waste heat. The standing waves can result in high voltage which can cause arcs in the applicator or waveguide, or may damage the generator. A change in load impedance can also affect the stability and performance of the generator. The simplest and most common approach to protect the amplifier is to provide a method of detecting excessive reflected power and reducing the power output proportionally (for example, see Jackson, et. Al. U.S. Pat. No. 5,084,425). This technique may prevent damage to the amplifier but has the disadvantage of preventing full power output.
A circulator device is usually installed in line at the generator output to absorb and dissipate reflected power which protects the generator and provides constant impedance. In some cases, load mismatch problems are partially addressed using manual or automatic waveguide tuners. These generally comprise of two or more tuning rods that are extended into the waveguide. Manual tuners cannot adapt to rapid or recurring changes in the load impedance. Automatic tuners are relatively slow because they are basically mechanical devices driven by stepping motors and both types add significant cost and complexity to the system.
Another approach is system tuning by varying the wavelength of the microwave energy. The wavelength is directly proportional to frequency. This is not possible with magnetrons because they are inherently fixed frequency devices. However, solid-state microwave generators can be frequency agile and operate anywhere within the permitted ISM bands. When the microwave frequency is swept across the band, impedance changes of the load become apparent. Often there will be multiple frequencies or ranges of frequencies where a relatively good match may occur. The invention provides a means to dynamically identify optimal matching frequencies and automatically tune the microwave generator to those frequency ranges. As a result, less reflected energy is absorbed by circulators. This saves energy and reduces the cooling system's required capacity.
Another crucial factor is distribution of electromagnetic energy in the three dimensional space within the applicator. This is a very complex topic. The electromagnetic field distribution depends on the microwave wavelength, the geometry of the applicator, the location, phase and type of the feed(s), and many other factors. The applicator may be of a pseudo-resonant type, which is referred to as a “single mode applicator” because a single electromagnetic mode is dominant within the applicator. Typically, there is an organized and well-defined area of high energy. This is the most desirable location for the load for maximum energy adsorption. Most commonly used are the “multimode applicators” which are non-resonant and have a wide variety of waves of various modes scattered in a somewhat random pattern. There is constructive and destructive interference of waves having different modes and phase at various points within the three-dimensional space of the applicator. This gives rise to “hot spots” and “cold spots” within the applicator. Often, cooking, drying, and heating systems are equipped with devices to stir the waves such as rotating antennas, or turntables used to rotate the load as are commonly provided in consumer microwave ovens. Practical applicator design is difficult. Powerful electromagnetic simulation software can be used to model applicator design, but real-world performance is often significantly different, usually due to dynamic load variation.
When the operating frequency of a microwave generator is swept across the band, the phase and modes of the intersecting waves in the applicator move around. As a result, interference nodes of high and low energy change locations in the three-dimensional space of the applicator. This results in more uniform heating of material inside the applicator, often eliminating the need for mechanical wave stirrers or turntables with a resulting reduction in cost and complexity.
The preceding explains two important benefits of the frequency agility of solid-state microwave generators. However, to take advantage of this ability it is also necessary to have precise-power control over a very wide dynamic range, and have highly accurate means to measure the forward output power of the generators and the energy reflected from the load due to impedance mismatch. It is also necessary to have a control system capable of automatic management of frequency and power during the operation of the entire microwave system. The control system also needs to be easy to operate for its users.